They bolted on a Stage 1 Weistec supercharger system that bumped power from the stock 350whp to a faintly terrifying 500whp. Along with the blower, it also included a new Euro-spec air box and mass air-flow sensor plus headers.

Now we’ve covered Weistec-supercharged cars in the past year (EC 12/11 and 6/12), so they don’t need a long intro. But to recap, the system is based on a 2.3-liter twin-screw supercharger with an innovative Constant Mu, Delta Pressure cog-drive system that helps maintain constant friction and boost pressure.

Unfortunately, 500whp wasn’t deemed sufficient to put it on a shortlist of the best C63s in the country, so Weistec’s Stage 2+ system was employed. In this iteration, the supercharger has a smaller cog, different headers as well as ported-n-polished heads with custom-ground 40mm intake valves and 33mm exhaust. You also get a bigger fuel pump, different spark plugs and re-mapped ECU.

For this machine, ACG went a step further by swapping the stock rods and pistons for Weistec’s forged, ceramic-coated, Teflon-skirted pistons and billet steel rods – apparently these were previously used in engines built to come closer to the 1000hp mark. But in this Stage 2+ configuration, the engine cranked out 693whp and 583lb-ft of torque on a recent Dynojet run.

It took just one brief test drive after the engine upgrade to come to the conclusion that the Mercedes was in need of two things: wider rear tires and bigger brakes.

The previous 19x9" rear wheels with 265/30 tires were simply overwhelmed by the engine output, sending the traction control into apoplexy and leaving the builders wondering how to fit wider tires.

Inevitably+, the only solution was to flare the rear fenders, with the cutting and fabrication taking about one week to complete. During that time, custom side skirts were also constructed to link both ends. The result looks factory fresh, with the flares maintaining the lines of the stock front fenders. But when compared side-by-side, the stock C63 looks anorexic and thin-waisted.

With so much extra room in the rear wells, ACG could fit 20x12" wheels and 325/35 Continental ExtremeContact DW tires to finally create some traction. Up front, 245/30s were wrapped around 8.5" wide wheels.

You might have noticed that thi Mercedes-Benz wears two different HRE wheel designs. On the left side are five-spoke C95s that are exclusive to ACG and can only be obtained through them. On the right are 797RS wheels in a split-five-spoke design as a further option. We’re not fans of this approach but it allows customers to visualize the different options, while the dimensions ensure both are eye-catching at least.

Behind the giant rims is a very expensive Brembo GTR brake kit. It’s the Italian company’s top-of-the-line system, comprising six-piston front calipers with enormous 405mm (15.9") two-piece rotors. On the rear are four-piston calipers squeezing 345mm (13.6") discs. Hopefully, this expensive solution has resolved the braking deficiencies, because there can be few options left, short of a boat anchor.

All the paintwork was expertly handled by Chapparone Auto Body, which included some surprisingly subtle red pinstriping. They also left some exposed carbon sections to highlight the weave underneath.

Inside, every surface has been re-done: the headliner and pillars covered in black suede, the dashboard and center console in leather with red stitching. MA Carbon pieces cover the doors, console, steering wheel and it uses their carbon shift paddles.

The front seats have red leather centers and AMG embroidered on the headrests, while the seatbacks have RevoZport carbon covers.

The audio system was also taken to the nines. Built around a TRU Technology Super Billet SB8-8 amplifier, it has a 12" Image Dynamix sub and Focal speakers all around. The system adds about 175 lb but the weight was kept low and near the wheels and act as ballast to keep the rear tires planted.

Driving it
As good as the sound system may be, the angry bellow from the long-tube MBH stainless headers, ACG link pipe and Akrapovic exhaust sounds even better. Sonically, it hits the sweet spot without overwhelming you. Distinctive at every RPM, you shift by listening to the tone.

Stage 2+ has a higher-pitched supercharger whine than Stage 1, but the faster-spinning supercharger means throttle response is noticeably sharper than the car we drove last year. Just the slightest nudge on the pedal will send the tacho needle soaring.

There’s pinned-to-the-seatback power from early in the rev range, and it builds quickly before riding a lofty plateau near the 700whp threshold. There doesn’t appear to be a noticeable drop-off as you pull the paddle for another gear, with each cog dispatched in what feels like a nanosecond.

The wide rear tires make the car feel planted and stable under hard acceleration, although credit should also go to the Quaife LSD as well as KW’s V3 coilovers. However, at times when we were in the 120s, the car started to wriggle as it tried to maintain composure over the small bumps and dips being thrown its way.

In the corners, turn-in was crisp as the body remained flat, reminding us of the CLK Black Series in its ability to control more than 4000 lb. Through mid-speed esses, the car was very stable, practically begging you to go faster.

We can debate whether this is the best C63 ad infinitum. Brabus and its 800hp C-class coupe or Bullit sedan would be the most tantalizing head-to-head match-up, but unfortunately we don’t know of one in the US.

That said, ACG’s C63 gets high marks for execution, and we’ve yet to drive another C63 with such devastating acceleration or composure in the corners.